Question

An aqueous solution of Mohr's salt can give a precipitate with $NaOH$ or ${NH}_{4}OH$. But, the aqueous solution of potassium ferrocyanide cannot give a precipitate with both reagents. How do you account for this?

• A. Both salts are double salts
• B. Both salts are complex salts
• C. First one is a double salt and other one is a complex salt
• D. First one is a complex salt and other one is a double salt

Answer 1

The correct option is C First one is a double salt and other one is a complex salt

$\left(N{H}_4{)}_{2}Fe\right(S{O}_4{)}_{2}6H_{2}O$ Mohr's salt is a double salt containing two cations and one anion, and is obtained by combination of two different salts $(N{H}_4{)}_{2}S{O}_4$ and $Fe\left(S{O}_4\right)$ which gives $N{H}_4\left(OH\right)$ and $Fe(OH{)}_2$ precipitates when added with $NaOH$ or $N{H}_{4}OH$. But $K_4\left[Fe\right(CN{)}_6]$ is a complex salt and do not form hydroxide with base.